1. Field of the Invention
Embodiments of the invention relate to methods for decreasing or eliminating electron beam damage to wafers in semiconductor fabrication processes and, more particularly, to methods for reducing electron beam damage to semiconductor wafers following W-CMP polishing of semiconductor wafers.
2. State of the Art
Critical dimensions of features on semiconductor wafers continue to shrink with advances in fabrication techniques. As the critical dimension sizes are decreased, defects associated with the fabrication processes increase. Although new fabrication processes are being developed to limit the formation of defects in semiconductor wafers during fabrication, defects still exist.
In a semiconductor wafer fabrication process, a semiconductor wafer undergoes many different fabrication process steps, including depositions, etching, polishing, rinsing, and the like. During these process steps, defects in the formation of the features of the semiconductor substrate may occur. Some defects in the semiconductor wafers result from corrosion. Corrosion may occur when semiconductor substrates are rinsed with an aqueous solution during the fabrication process. For example, semiconductor wafers having metallization features may undergo one or more rinsing processes wherein the semiconductor wafer is rinsed or washed in an aqueous solution. Contact between the aqueous solution and metals of the semiconductor wafer may result in the corrosion of the metal on the semiconductor wafer. Such corrosion may cause a defect in the semiconductor wafer.
Different forms of corrosion have been associated with semiconductor wafer fabrication. In some instances, the corrosion of semiconductor features may result from corrosion caused by the process chemistry. In other instances, corrosion may result from the rinsing of semiconductor wafers in de-ionized water, which is conventionally performed in semiconductor fabrication processes. For example, direct dissolution may occur where metal in the semiconductor substrate is directly dissolved by de-ionized (DI) water during a semiconductor substrate rinse process. Such corrosion, however, is rare in semiconductor substrates because the metals used in semiconductor processes are not typically prone to such dissolution. Concentration cell corrosion occurs where there is a substantial change in the pH of solutions contacting the semiconductor substrate. For example, concentration cell corrosion may occur when a semiconductor substrate is exposed to a solution having a high or low pH followed by exposure to a DI water rinse having a relatively neutral pH. Galvanic corrosion occurs when two different metals are simultaneously exposed to an electrolyte. Although pure DI water rinses are poor conductors of electrons, residual process chemicals carried into a DI water rinse bath on the surfaces of a semiconductor substrate may be sufficient to act as an electrolyte and promote corrosion between dissimilar metals on the semiconductor substrate. Process chemicals carried into a pure DI water rinse bath on the surfaces of a semiconductor substrate may also create an environment within which corrosion is promoted. For instance, process chemicals may react with DI water to promote the formation of hydroxyl ions (OH—) that may then react with metals in the semiconductor substrate surfaces.
To help identify defects in semiconductor wafers, such as damage caused by corrosion, electron beam inspection instruments are used to inspect and review wafers throughout semiconductor fabrication processes. Electron beam inspection instruments, such as scanning electron microscopes and variations thereof, may be employed with semiconductor fabrication processes to inspect semiconductor wafers after particular fabrication processes. The electron beam inspection instruments use electron beams to create voltage contrasts between device structures to enhance or identify the conductivity of various portions of a semiconductor wafer. The identification of the conductive and less conductive portions of a semiconductor wafer helps to identify defects in the semiconductor wafer. For example, electron beam inspection instruments may be used to examine tungsten contacts on a semiconductor wafer after a tungsten chemical mechanical polishing step during semiconductor wafer fabrication. Tungsten metal fills openings, or vias, in the semiconductor wafer and connects with underlying structures. If a via is not sufficiently filled with tungsten or conductive metal, poor electrical continuity may result, indicating that errors in the fabrication of the semiconductor wafer may have occurred. The electron beam inspection instruments may be used to identify vias or contacts having insufficient tungsten deposition and insufficient conductivity, thereby identifying errors or defects in the semiconductor wafer.
Electron beam inspection instruments identify errors or defects in the semiconductor wafers by analyzing the surface electrons of the semiconductor wafers. The electron beam inspection instruments disrupt electrons on the surface of the semiconductor wafers, allowing detection or visualization of the surface being analyzed, such as by using voltage contrast measurements of the surface of the semiconductor wafers. In some instances, the electron beam inspection instruments extract, or pull, electrons from the surface of the semiconductor wafers, leaving the surface positively charged. In other instances, the electron beam inspection instruments implant electrons onto the surface of the semiconductor wafers, retarding the surfaces and leaving a negative charge on the surface of the semiconductor wafer.
While electron beam inspection instruments are employed to detect defects in semiconductor wafers, it has also been discovered that the irradiation of the semiconductor wafers by the electron beam inspection instruments may cause damage to semiconductor wafers. The damage to semiconductor wafers caused by the electron beam inspection instruments is undesirable.
Therefore, it would be desirable to develop methods for reducing corrosion on semiconductor wafers during fabrication processes. It is also desirable to reduce or eliminate damage to semiconductor wafers caused by electron beam inspection processes.